Printed circuit substrates are employed in the electronic industry for the placement and interconnection of electronic circuits. Examples of printed circuit substrates in use in the electronic industry include: ceramic substrates, printed circuit boards, flexible printed circuits, porcelain-on-metal substrates, and silicon-on-silicon substrates. These electronic circuits may comprise expensive microprocessor or digital signal processor circuitry in large packages requiring many pinouts to connect to the printed circuit substrate. For example, a ball-grid array (BGA) package may be used for such circuitry. These BGA packages are typically electrically connected to a printed circuit substrate by a surface mounting technique, such as a mass solder reflow operation using hot air or other hot gas such as nitrogen.
The BGA package in particular causes some manufacturing difficulty as the solder connections from the printed circuit substrate to the BGA pinouts are directly beneath the BGA package. Therefore, standard solder wave reflow technique can not be used as the solder can not move beneath the package to make the connections. Moreover, the density of these connections can be quite large results in a very small pitch between connections which can result in solder bridging that electrically shorts connections together. As a result, special assembly techniques must be used that can include solder balls, printed solder paste, or solder bumps pre-placed on the circuit substrate and spaced apart by a solder mask. The BGA package is then placed on the paste, solder bumps or balls, and the assembly is heated using hot air or other hot gas such as nitrogen to melt the solder in order to connect the BGA pinouts to the printed circuit substrate connections. While most BGA attach and repair techniques use hot air to melt the solder, for single sided assemblies, sometimes a hot plate or heated cartridge under the circuit substrate is used to melt the solder. Other means of locally heating the circuit substrate that have been used to locally melt solder include: infrared radiation, soft beam light energy, laser light energy, and applying a hot condensing vapor such as fluorinert (as is used in vapor phase reflow systems).
The problem becomes worse in the removal of defective BGA packages, in that, the removal process typically involves reheating the solder connection and carefully removing the BGA device. If the BGA device is not lifted off the board properly (i.e. a lateral movement occurs) solder bridging can occur requiring further recovery measures or the scrapping of the circuit substrate. In addition, the solder mask used between connections is delicate and flexible and can be damaged if any lateral movement occurs in the removal of the BGA package. If the solder mask is damaged the re-installation of a new BGA device on the reworked circuit substrate can easily result in a new solder bridge, which then requires further rework.
Prior art methods include using a cutting wire to remove the package and the use of high-pressure water jets. However, these mechanical techniques will definitely cause damage to the solder mask. Reflowing the solder to remove the part is least damaging to the solder mask. However, there may be an excessive amount of residual solder left over after the removal of the BGA package, which requires further rework on the circuit substrate, such as using a braided copper solder wick as is known in the art. This further rework can damage the solder mask, also.
Therefore, a need exists for an improved technique for the removal of excess solder, which can result in solder bridges, from a circuit substrate without damage to a solder mask. It would also be desirable to provide such improvement in a simple, single gang operation that limits the potential for damaging lateral movement. It would be of further benefit to use the same equipment for connecting and for disconnecting circuits to the printed circuit substrate, so as to make the removal technique economical and to maintain process control.
While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents and alternatives falling within the broad scope of the invention as defined by the appended claims.